1. Field of the Invention
The present invention relates to a transmission mounted in a vehicle, and in particular, relates to a case structure for a transmission.
2. Related Art
An automatic transmission mounted in a vehicle conventionally includes a hydraulic power transmission such as a fluid coupling that generally serves as a clutch and a torque converter, and a shifting mechanism in the form of a planetary gear set, a hydraulically-operated clutch, and a brake. The hydraulic power transmission and shifting mechanism are respectively accommodated within a converter housing and a shifting mechanism (gear) case. The shifting mechanism case interior serves as a space forming a portion of an oil passage for circulating automatic transmission fluid (ATF) for lubricating and cooling all parts of the shifting mechanism and, therefore, is an oil-tight space with respect to the exterior of the transmission. The converter housing covers only the outer side of the hydraulic power transmission, and serves as a structural member connecting the automatic transmission to the engine. Therefore, the interior space housing the shifting mechanism is separate and isolated from the interior space of the converter housing. The interior spaces of the shifting mechanism case and the converter housing are normally mutually isolated by the body of an oil pump serving as a partition wall. The pump itself is a hydraulic pressure source for supplying transmission hydraulic oil to each of the hydraulic power transmission and the hydraulic servos that operate the brakes and clutches of the shifting mechanism, and to other portions of the shifting mechanism.
The automatic transmission case mounted in a vehicle (as used herein “automatic transmission case” has reference to, collectively, both the converter housing and the shifting mechanism case), especially an automatic transmission case vertically mounted in a FR (front engine rear drive) vehicle, has a shifting mechanism case with an oblong and substantially cylindrical shape and, therefore, has relatively low strength in comparison to the converter housing that has a truncated cone shape with a short axial length and which, therefore, is a portion with greater strength by nature. Accordingly, conventionally, in order to ensure the strength of the shifting mechanism (gear) case, measures for ensuring strength by increasing the thickness of the case wall and providing ribs have been adopted.
Strength maintaining measures such as the above are effective for preventing deformation and damage to the automatic transmission case itself due to loads from vibrations and centrifugal force during normal vehicle travel. However, when the automatic transmission case is considered as a component of the drive train located between the engine and the propeller shaft, particularly, in the case of receipt of an extremely large offset impact load, it is extremely difficult for the automatic transmission case to endure such an impact. Maintaining strength against such an impact load by conventional measures, such as increasing wall thickness, have limitations due to reasons such as the occurrence of shrinkage during case casting and extreme increases in weight. But even if such problems could be resolved, depending on the size of the impact load, the conventional approach would not necessarily guarantee the prevention of damage. The same can also be said of a manual transmission case of an identical structure including a clutch housing enclosing a dry clutch that is provided in a front portion of a shifting mechanism (gear) case.